1. Field of the Invention
The present invention relates to a communication system (FFH-OFDM communication system) using a Fast Frequency Hopping (FFH) scheme and an Orthogonal Frequency Division Multiplexing (OFDM) scheme. More particularly, the present invention relates to a method and an apparatus for transmitting/receiving signals by using only a subset of all of the available frequency bands used in an FFH-OFDM communication system.
2. Description of the Related Art
In a 4th generation (4G) communication system, research has been actively pursued to provide users with services having various qualities of service (QoS) and supporting a high transmission speed. Additionally, research has been actively pursued to develop a new communication system capable of supporting high speed services, and ensuring mobility and QoS in a wireless local area network (LAN) and a metropolitan area network (MAN) system.
In order to support a broadband transmission network for a physical channel of the wireless MAN system, the OFDM scheme and an Orthogonal Frequency Division Multiple Access (OFDMA) scheme are being widely utilized. The OFDM/OFDMA scheme can achieve high speed data transmission by using a plurality of sub-carriers when transmitting physical channel signals, and can acquire a frequency diversity gain by using different sub-carrier bands for frequency bands for signal transmission.
In comparison to a system using a single sub-carrier, the OFDM communication system which uses a plurality of sub-carriers has a symbol period that is prolonged in proportion to the number of sub-carriers with respect to transmission speed for the same data. By using a guard interval, the OFDM communication system can reduce the Inter-Symbol Interference (ISI) in a wireless channel having multipath fading. Methods for inserting the guard interval include a cyclic prefix method and a cyclic postfix method. In the cyclic prefix method, a predetermined number of last samples of an OFDM symbol in a time domain are copied and inserted into a valid OFDM symbol. In the cyclic postfix method, a predetermined number of first samples of an OFDM symbol in a time domain are copied and inserted into a valid OFDM symbol.
Further, a channel response of each sub-carrier band is approximated to be flat in the sub-carrier band. Also, by setting the difference Δf between sub-carrier frequencies to be an inverse multiple the sampling period T, (Δf=1/Ts), it is possible to reduce the Inter-Carrier Interference (ICI) during one OFDM symbol period. When the sub-carriers are orthogonal to each other and there is no interference between them. The receiver of the OFDM communication system can demodulate data by using a single tap equalizer which has a relatively simple structure. Further, the OFDM communication system can minimize the complexity of the system, since the OFDM communication system uses an Inverse Fast Fourier Transform (IFFT) scheme and a Fast Fourier Transform (FFT) scheme in order to modulate/demodulate the plurality of sub-carriers. An operation of an Inverse Fast Fourier Transform unit (IFFT unit) using the IFFT scheme corresponds to a frequency modulation operation in the OFDM communication system, which can be expressed by a matrix DQ as defined by equation (1) below.
                                                        [                                                D                  _                                Q                            ]                                      n              ,              m                                =                                    1                              Q                                      ⁡                          [                              exp                ⁢                                  {                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                                                                  π                        ⁡                                                  (                                                      n                            -                            1                                                    )                                                                    ·                                                                        (                                                      m                            -                            1                                                    )                                                Q                                                                              }                                            ]                                      ,                                  ⁢        m        ,                  n          =          1                ,        …        ⁢                                  ,        Q                            (        1        )            
In equation (1), Q denotes the number of all sub-carriers used in the OFDM communication system, n denotes a sample index, and m denotes a sub-channel index. DQ denotes an IFFT matrix having a size of Q×Q. The sub-channel refers to a channel including at least one sub-carrier. Further, an operation of a Fast Fourier Transform (FFT) unit using the FFT scheme can be expressed by a Hermitian DQH of the IFFT matrix DQ as defined by equation (1).
Meanwhile, when the plurality of sub-carriers in the OFDM communication system include sub-carriers which undergo deep fading, there is a low probability that data transmitted by the sub-carriers undergoing the deep fading can be successfully decoded by the receiver. Schemes proposed in order to overcome performance degradation due to the deep fading include a frequency hopping scheme and a Forward Error Correction (FEC) scheme.
The frequency hopping scheme is a scheme in which the frequency band for signal transmission is changed according to a predetermined frequency hopping pattern, so that it is possible to obtain an average gain of the Inter-Cell Interference (ICI). In other words, according to the frequency hopping scheme, signals are transmitted while transmission frequency bands for sub-carriers are periodically changed according to the predetermined frequency hopping pattern. Therefore, the frequency hopping scheme can prevent signals from being continuously transmitted to a single user by the sub-carriers undergoing the deep fading due to the frequency selective channel characteristic. The frequency hopping period is a time interval corresponding to an integer number of times of the OFDM symbol time or OFDM symbol. As a result, in the case of using the frequency hopping scheme, even when signals are transmitted by sub-carriers undergoing the deep fading at a predetermined OFDM symbol time, it is possible to transmit the signals by sub-carriers which do not undergo the deep fading at a next OFDM symbol time, thereby preventing the signal transmission from being continuously influenced by the deep fading, so that it is possible to average the frequency diversity gain and interference.
Further, according to an FH-OFDM scheme which is a combination of the frequency hopping scheme and the OFDM scheme, different sub-channels are allocated to users, and the sub-channels allocated to the users are subjected to frequency hopping. This makes it possible to obtain the frequency diversity gain and ICI average gain.
However, in order to have a sufficient gain by the frequency hopping scheme, the conventional OFDM communication system must perform the frequency hopping during many OFDM symbol time periods. Further, it requires a large number of users, and must select a proper frequency hopping pattern according to the channels. Even further, in the conventional OFDM communication system using the frequency hopping scheme, although signals are not continuously transmitted to a single user by the sub-carriers undergoing the deep fading, it is still impossible for a receiver to demodulate the signals transmitted by the sub-carriers undergoing the deep fading at every OFDM symbol time period.
Accordingly, there is a need for an improved method and an apparatus for transmitting/receiving signals by using only a subset of all of the available frequency bands used in an FFH-OFDM communication system so as to enable a receiver to demodulate signals transmitted by the sub-carriers undergoing deep fading.